This project is aimed at studying further the structural details of the membranes forming low resistance junctions between cells of normal and neoplastic tissues, in an attempt to establish more precise structure-function correlations. A portion of the project is based on the hypothesis that variations in the arrangement of certain globular structures seen in membranes of crayfish axo-axonal junctions are paralleled by changes in membrane resistance. This hypothesis will be tested in lanthanum treated preparations by studying the junctions of cells uncoupled by various methods, in an attempt to define whether or not variations in membrane permeability can be meaningfully correlated with structural changes. In addition, a detailed study of the structures forming two globular patterns previously seen in membranes of the junctions will be performed on crayfish ganglia prepared by fixation methods utilizing glutaraldehyde-H2O2 and increased pH (Peracchia and Mittler, 1972a,b) or by freeze-etching. Phe same structures will be studies in isolated junctions prepared by freeze-etching and negative staining associated with optical diffraction analysis. The information to be obtained will include number, size, shape, and organization of the subunits composing each globular unit in the patterns and possibly the presence of presumed filaments bridging adjacent globules. The findings will allow one to test a hypothetical model proposed to explain the plasticity properties of the structures forming the junctional membranes. Other studies will be aimed at learning whether or not changes in the fine structures of the globular arrays are detectable in membranes of low resistance junctions between malignant cells versus those between normal cells. For this purpose the structure of junctions between electrically coupled cells from neoplastic tissues growing in vitro or in vivo will be studied by the various morphological methods previously described.